This invention pertains to the field of embedded sector servo technology and more specifically, to synchronous dithering of a disk drive position error signal for demodulation.
In direct access storage devices, such as disk drives, which employ embedded sector servo technology, burst fields are written to each servo field of a disk surface. Each burst field is written with a constant frequency signal. The burst fields are typically staggered so that the amplitude of burst field signal measured by a reading head over a servo field is useful for indicating any error in the positioning of the head over a track. A detected burst field signal is referred to herein as a Position Error Signal (PES). The pattern of PES amplitudes measured by a reading head passing over a servo field of a track, together with the pattern of burst field locations on the track, and on adjacent tracks, indicate the magnitude and direction of any error in the lateral position of the head.
Many modern disk drives use some form of digital signal processing to close a servo loop, in order to place a head at a specific location. The servo electronics of a disk drive are required to accurately read the amplitude, and possibly the phase, of the PES in order to supply the digital signal processor with digital values which are necessary to center the head above the desired track on a disk. The conversion of an analog PES to a digital PES value useful for determining the position error is referred to as PES demodulation.
One measure of performance for a disk drive is how close two data tracks can be placed together. This track density parameter is one of the most important parameters in determining the total data capacity of a disk. Some loss in track density results if the PES demodulator does not provide a sufficiently accurate digital PES value.
Another measure of performance for a disk drive is the head seek performance, which is a measure of the speed with which the drive can accurately place a head over a desired track, before reading from or writing to the track. Because faster head seek performance reduces the latency of the drive and increases the overall data throughput, high-performance disk drives typically have a faster head seek performance than lesser drives. High speed track seeking and following capability requires some form of predictive capability. Accurate prediction is achievable only if the digital PES value is accurate. Accurate PES demodulation is thus important for high-performance disk drives.
Analog synchronous PES demodulators are among the most accurate demodulators used today. An analog synchronous PES demodulator synchronously multiplies the PES with a reference clock square wave in a process known as xe2x80x9csynchronous choppingxe2x80x9d, to obtain a rectified PES. The reference clock is generally derived from another burst field, known as the sync-field, which has the same frequency and phase as the PES burst fields. The rectified PES is then integrated over a fixed number of burst cycles. The integrated value corresponds to the magnitude of the PES.
Precision analog synchronous PES demodulators are difficult to implement, and often require large, complex circuitry. Integration of an analog synchronous PES demodulator into the main data channel of a disk drive is highly desirable, but is difficult to achieve due to the size and complexity of the circuitry. Current implementations of stand-alone analog synchronous demodulators are almost as large as the main data channel. Cost reduction is likely achievable through the use of a digital PES demodulator implementation.
In modern disk drives, the main data channel usually contains a 6 bit Analog-to-Digital Converter (ADC) for data detection. In some high-performance disk drives, digital PES resolution of more than 9 bits is necessary for adequate position error correction. A digital synchronous demodulator has been described for detecting phase-encoded servo signals. See U.S. Pat. No. 5,343,340 to Boutaghou et al. A thorough numerical modeling of such systems, however, reveals that even a 7 bit ADC would result in only slightly better than 7 bit digital PES resolution.
It would be desirable to provide a digital PES demodulator which takes advantage of the ADC present in the main data channel of a disk drive to provide a high resolution digital PES for position error correction.
The effective resolution of an integrated digital PES value is increased by synchronously dithering the analog PES before sampling by the read channel ADC. For each burst cycle of an integration window, a DC dithering offset is added to the analog PES before sampling. The dithering offset is changed periodically, during the integration window. The dithering offsets are evenly spaced over a range which is equal to the product of the size of the least significant bit of the ADC multiplied by a positive integer, resulting in enhanced resolution of the digital PES.